1. Field of the Invention
The invention relates to a protective device for low-voltage lines which can be fitted to a cable connection head for connecting a cable of telephone lines to the switching center of these lines, such as a telephone exchange. This device is designed to protect the lines against destructive electrical disturbances such as over-voltage caused by lightning discharges, by electric power transmission lines or, more generally, by high voltage sources.
2. Description of the Prior Art
A protective device for low-voltage lines is disclosed in the U.S. Pat. No. 4,146,755. The cable connection head onto which the protective device is fitted, is of the type having separable leaf-spring contact junctions, ends of which are constituted by pins which are connected, before fitting, to the wires of the cable and of the exchange, respectively. First conductive means associated with a pair of junctions of a line includes a two-sided printed plug sliding within the body of the protective device. This plug carries two pairs of conductive strips of which one on one side connects, after fitting, first terminals of the two protective circuits to the two associated exchange wires through two corresponding leaf-spring contacts of the two associated junctions, and of which the other connects, after fitting, second terminals of the two protective circuits to the exchange ground through the two other leaf-spring contacts of the two associated junctions and adequate connecting means of the head and the protective device. Insertion of the plug separates the leaf-springs of two pairs in order to interconnect two overvoltage protectors. When fitting the protective device to the cable head, the cable wires are no longer connected to the associated pins or leaf-spring ends of the junctions but to two particular pins of the protective device connected through a fuse i.e. an overcurrent protector, to the leaf-springs of the junction still connected to the exchange wires. It appears that, in order to fit the device to the head, it is necessary on the one hand to disconnect the cable wire of each line from the end pin of the associated junction contact leaf-spring and to connect this cable wire to a suitable pin on the device and, on the other hand, to connect the leaf-spring pin, previously connected to the cable wire, to the exchange ground.
For example, when putting electronic telephone exchanges into service, while retaining the system of subscriber's loop cables, the installation of such protective device requires numerous wiring, unwiring and rewiring operations at the exchange frame. This involves high installation costs as well as a temporary and disturbing unavailability of the lines.